Simplify the following expression: $a = \dfrac{3}{4r} + \dfrac{9}{10r}$
Solution: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4r$ and $10r$ $\lcm(4r, 10r) = 20r$ $ a = \dfrac{5}{5} \cdot \dfrac{3}{4r} + \dfrac{2}{2} \cdot \dfrac{9}{10r} $ $a = \dfrac{15}{20r} + \dfrac{18}{20r}$ $a = \dfrac{15 +18}{20r}$ $a = \dfrac{33}{20r}$